1. Field of the Invention
The present invention relates generally to data transmission links and in one of its aspects to a fiber optic data transmission link to be powered by low voltage single polarity power supplies with unreliable voltage levels over a wide range of temperatures.
2. Description of the Prior Art
Control systems such as control systems for airplanes are often of a conventional wire connected electrical nature. Wire connected electrical control systems are subject to electrical noise picked up by the control cables. Other conducted electrical data transmission links are subject to similar problems. One way of obviating the problem of electrical noise picked up by the transmission cables is to transmit data optically rather than electrically. One way of transmitting optical data is by the use of fiber optics cables.
One way of generating optical data is by the use of a light emitting device such as a light emitting diode known as an LED. The LED is a low impedance device when operated under forward bias and exhibits a negative temperature coefficient. Thus, to avoid thermal runaway, the LED can be driven from a high impedance source, ideally a constant current source. Control systems are often used in environments where the power supply is low voltage and produces output with an unreliable voltage level, and temperatures vary over a wide range, so the LED transmitter must regulate the LED current against changes in supply voltage and changes in temperature.
Most amplifier work in the past has been done in "matching systems", systems where the transmitting medium such as a cable or an antenna is largely resistive in nature so that the transmitters and receivers are designed to match the impedance of the cable or antenna. Photodiodes, on the other hand, are highly capacitive in nature.
In order to put an entire transmitter or an entire receiver upon a single chip, it is necessary to make the transmitter or receiver and the logic of compatible integrated circuit technology. Typically the logic circuitry is transistor-transistor logic known as TTL. TTL is almost always used to transmit unipolar signals, but data is often most easily transmitted by a bipolar system, especially for transmitting codes with approximately a 50 percent duty cycle such as a Manchester code since the problems of dc voltage offsets can be avoided.
A common restriction, especially for systems designed for the military, is that the system be able to operate on a positive power supply only. Additionally, most of the bipolar technology industry is geared for producing npn technology only.
There is very little problem with noise picked up in transmission in fiber optics systems, so that the noise generated from the system itself determines the performance of the system.